There are so many word variations in the English language that many people miss the subtle difference between "forty" and "fourty." However, that difference is there, and any spell-checking software will tell you that "fourty" is a no-no. (The one we're using is doing exactly that right now!) The question, however, is why? Where along the well-beaten path of our language did that "u" disappear? Was it taken away due to some nefarious purpose, cast into the dungeon of unnecessary letters? And why do both "fourteen" and "fourth" still have their "u" while "forty" doesn't? One needs to go back a few hundred years to merry old England to find out.
Forty has been a popular number for hundreds, no, thousands of years. In the 12 century B.C., the word feowerti meant forty, which was derived from the word feowertig. Going back even further, the proto-Indo-European word kwetwer was what one used when they wanted to convey that, for example, they had forty cows or forty hectares of land.
Still, none of those examples were even close to the word we use today, especially since they were missing both the "o" and the "u" (let alone the "y.") Nevertheless, sometime during the 16th to 18th centuries, the word for 40 was most commonly "fourty," which means that, if you make a mistake with the spelling, you were right a few hundred years ago.
Some writers and even some publications still make the mistake today. For example, Nature World News wrote this in 2015: "Female grey whales are fighting for their life. Fourty-three female whales are breeding in the group in 2015, a big increase from the 27 female whales in 2004." In 2016, Empire Advance, a local newspaper out of Manitoba, Canada, wrote this little tidbit in their online publication: "Fourty years ago, when the mayor of Elkhorn was Bill Bartley, he and his council members saw the need for a village seniors complex and residence that would keep area seniors in the community as active members." Although published, both of these examples are 100% wrong.
Some have surmised over the years that fourty is simply the English spelling of the word forty, but they would be wrong. In both countries, the correct spelling is forty, even though the word colour in the U.K. uses a "u," as do the words inferiour, mirrour, horrour, and errour, among others. Why the extra "u" in Great Britain? Because in the 17th century, the Old Bailey (a London court) ruled that -our endings were the correct way to spell words with suffixes of Greek and/or Latin origins. Forty, however, has neither Latin nor Greek origins and thus doesn't use the "u."
If you're wondering whether using "fourty" is correct when writing about money, it's not. (Sorry.) The correct spelling is always forty dollars and never fourty dollars (and would make a mess if you wrote "fourty" on a check or an online payment portal). Four dollars would be correct, of course, as well as fourteen dollars and fourteen cents, but fourty dollars would make your banker do a double-take and question your financial intelligence.
This next example is one that trips up many people, but as with all of the other examples, there's only one correct way to spell it. That way would be "forty-five" with a hyphen. Fourty five is incorrect, and, surprisingly, forty five without a hyphen is also incorrect. That little guy must be there, or you'll get the stink eye from your teacher, accountant, or other financially-invested people.
The reason is that 45 is a compound number between the numbers 21 and 99. As such, proper English demands it has a hyphen when written out. The only compound numbers that don't require a hyphen are those divisible by 10. So twenty, thirty, forty, etc. In other words, from forty-one through forty-nine, all need that little hyphen to be 100% correctly spelled.
Furthermore, in the debate between forty one or fourty one, neither would be correct. Forty-one would be the true, correct spelling. The same goes for forty five or fourty five. Both are wrong since forty-five, as a compound number, is the only correct spelling.
If you're wondering what the dictionary has to say about forty and fourty, every dictionary agrees; forty is the correct spelling. The Merriam-Webster dictionary, for example, says that "forty" is correct. Dictionary.com agrees and says that forty is the only way to spell this compound number. The Cambridge Dictionary also agrees that, in the "fourty or forty" debate, forty is the correct spelling at all times and under all circumstances.
Wondering what American history has to say about the debate between forty and fourty? Some of the best examples come from the early to mid-1800s. Although coined by an Englishman (William Kitchiner M.D.) around 1821, the phrase forty winks is used by many Americans still today. (It means to get a quick nap.) During the gold rush in California in 1849, the word for the adventurous men who tried to strike it rich was forty-niner (notice the hyphen). That example was so strong that, even today, the San Francisco Forty-Niners are one of the NFL's most fabled football teams.
As we've seen, there's no debate between which is correct, forty of fourty. Although fourty might have been widely used for a time hundreds of years ago, it's long been held that forty is the correct and only spelling for the number 40. We've also seen that, when it comes to the fourty five or forty five debate, both are wrong since forty-five, with a hyphen, is the only way to spell this compound number.
How can you know which is correct when you're writing? Easy. Every iteration of "four" except forty has a "u." (Four, fourth, fourteen, four thousand, etc.) So memorize that fact, and you'll be just fine! Forty is the only way to spell it, and that's that! (Even if the occasional writer or publication makes a mistake.)